The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming more and more common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming signal.
Spread spectrum communications systems are commonly used in military environments to overcome high energy narrowband enemy jamming. In commercial or home environments it may be used to achieve reliable communication on noisy media such as the AC powerline. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the powerline. For example, electronic dimming devices can place large amounts of noise onto the powerline since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC powerline may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
One common type of spread spectrum communications, called direct sequence spread spectrum, is generated by first modulating the digital data and then multiplying the result with a signal having particularly desirable spectral properties, such as a PN sequence. The PN sequence is a periodic sequence of bits having a period N. Each bit in the sequence is termed a chip. The sequence has the property of having very low autocorrelation for delays larger than one chip. In some systems, the PN sequence is replaced by a chirp waveform.
The spread spectrum receiver is required to perform synchronization which is commonly implemented using an acquisition method in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC powerline, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive. In addition, these systems typically succeed at transmitting only one or two bits per symbol.